The present invention relates generally to solar cells and/or other light-to-electrical energy transducers; and, more particularly, to solar cells--especially, concentrator solar cells--and to methods of manufacture thereof, characterized in that the cells have (i) a relatively deep n.sup.+ -p or p.sup.+ -n junction (on the order of 0.5 .mu.m or greater) with high surface concentrations of dopant in the near-surface regions where electrodes are to be formed by, for example, vacuum deposition and/or plating techniques, and (ii), a relatively shallow junction (on the order of 0.3 .mu.m.+-.0.1 .mu.m) in the inter-electrode near-surface regions defining the cell's photoactive regions, which inter-electrode regions are preferably texturized; thereby providing cells characterized by: (a) their low resistance and excellent ohmic contact properties immediately adjacent the electrodes; (b) high current generating and low reflective loss properties in the photoactive regions of the cell; and (c), relatively high thermal stability.
One of the more perplexing problems faced by designers, manufacturers and users of light-to-electrical energy transducers such as solar cells and concentrator solar cells has, for a number of years, been, and continues today to be, the need to improve both the light energy collection efficiency and the conversion efficiency of light to electrical energy. In this connection, it has long been recognized that light reflected from the face of a solar cell is a principal source of poor light collection efficiency, and many efforts have been made, and are continuing to be made, to solve this problem. Initially, such efforts were primarily directed towards providing a thin, non-reflective, transparent barrier layer; but, such non-reflective barrier layers, of and by themselves, have not provided a satisfactory solution to the problem. Other efforts have included deformation of the photoactive face of the cell itself so as to form a multi-faceted photoactive face wherein light is reflected from one facet to another so as to improve collection efficiency. Typical examples of this latter approach are suggested in Lamb U.S. Pat. No. 2,320,185 (wherein a photosensitive oxide layer is applied to an irregular, die stamped, copper blank or, wherein a photosensitive selenium layer is subjected to pressing to form a multi-faceted surface); and, in Rudenberg et al U.S. Pat. No. 3,150,999 (wherein a block of semiconductor material--e.g., silicon--is placed in a "waffle iron" type die and subjected to ultrasonic vibration). Rudenberg et al also suggest that the desired multi-faceted faces can be obtained by etching.
In addition to the etching suggestion contained in the aforesaid Rudenberg et al patent, there has been a wealth of work in the area of etchants and etching processes to form a "texturized" surface on a photocell substrate--viz., a surface characterized by randomly located irregularities (commonly pyramidal in shape) defining light absorptive surfaces having reflecting facets to improve collection efficiencies. For example, in McKelvey et al Canadian Pat. No. 751,084 (1967), the patentees disclose a process for forming a texturized photoactive surface on an n-type substrate by first etching the surface with an aqueous hydroxide solution--e.g., sodium hydroxide or potassium hydroxide. Thereafter, the textured surface has a diffused layer of impurity atoms formed therein in a conventional manner; such diffusion layer defining a p.sup.+ -n junction stated to be about "one micron" (1.0 .mu.m) in thickness. Following formation of the diffused layer, the patentees apply a first ohmic contact directly to the texturized surface and a second ohmic contact on the back of the n-type substrate. See, also, Bailey et al U.S. Pat. No. 4,137,123 which discloses a surface etchant for silicon comprising an anisotropic etchant for producing a texturized surface defined by a random distribution of minute pyramids.
Rittner U.S. Pat. No. 4,135,950 discloses an arrangement for forming V-shaped grooves having a depth of 320 .mu.m and a width of 416 .mu.m by first masking a p-type silicon substrate to preclude formation of a multiplicity of randon pyramids and then etching the unmasked surface with hydrazine. Thereafter, a thin n-type layer is diffused into the V-shaped grooved surface and electrodes are applied to the mesa-like peaks between adjacent grooves. Thus, the arrangement is such that the hydrazine etching process is controlled by masking to preclude the formation of a texturized surface defined by a multiplicity of randomly located pyramids while permitting the formation of a plurality of elongate, parallel, deep V-shaped grooves.
The concept of masking the substrate and thereafter preferentially etching the unmasked surface to produce a "texturized" photoactive surface is one that has been conventionally disclosed and used in the prior art. For example, such an arrangement is disclosed in an article entitled "Total Photon Absorption Solar Cells" by J. J. Cuomo and L. Kuhn, IBM Technical Disclosure Bulletin, Vol. 18, No. Aug. 3, 1975, as well as in Magee et al U.S. Pat. No. 4,147,564, see, esp., Magee et al FIGS. 4A and 4B.
Other representative prior art patents of miscellaneous interest include, for example: Gereth et al. U.S. Pat. No. 3,686,036; Chiang et al U.S. Pat. No. 4,133,698; Avery et al. U.S. Pat. No. 4,158,591; Longshore U.S. Pat. No. 4,160,045; and, Bube U.S. Pat. No. 4,163,678--although such patents do not relate to the particular double or two step diffusion process with an intermediate etch to entirely remove selected portions of the deep diffusion zone as employed with the present invention.
In general, the specific types of dopant employed in a diffusion process or, indeed, the particular process employed for dispersing impurity atoms into the near-surface regions of a substrate, can vary widely dependent upon such factors as, merely by way of example: the material from which the substrate is formed--e.g., silicon, selenium, or other well-known materials; whether the substrate is a p-type or an n-type material; whether the impurity atoms are to be, e.g., phosphorous, boron, arsenic, antimony, etc,; the depth of the junction to be formed; whether a conventional gas diffusion process is to be employed to disperse or diffuse impurity atoms in the substrate, or whether an ion implantation process is to be employed wherein impurity atoms are dispersed in the substrate by ion bombardment; etc. However, irrespective of the particular process employed, it has generally been known that it is desirable to form a deep junction--preferably on the order of 0.5 .mu.m, or greater, in depth--with high surface concentrations of dopant in the regions immediately beneath the surface electrodes so as to insure good ohmic contact. At the same time, it is also known that optimized current generation in the photoactive region of the solar cell mandates the formation of a relatively shallow junction--preferably on the order of only 0.3 .mu.m.+-.0.1 .mu.m in thickness--having lower surface concentrations of dopant, with the shallow junction preferably being formed on a texturized surface of the type disclosed, for example, in the aforesaid Rudenberg et al, McKelvey et al., Bailey et al. and Magee et al. patents and/or the Cuomo et al. article. Such disclosures, however, do not deal with, or even recognize the need for, differential diffusion depths and/or differential near-surface impurity concentration levels in the photoactive regions of the cells and in the regions under the exposed surface electrodes.
The concept of a solar cell having both deep and shallow junctions has, however, been disclosed in Matsutani et al. U.S. Pat. No. 4,029,518, as well as in Gonsiorawski U.S. Pat. No. 4,152,824. Matsutani et al. discloses two processes for obtaining the desired differing depth junctions. In the first process (FIG. No. 2 of Matsutani et al.), a mask is applied to the photoactive regions of the substrate and a diffusion process is employed to form a deep junction on the order of 3.0 .mu.m only in the unmasked regions (i.e., the regions where the electrodes are to be formed). The mask is then removed and a shallower junction--e.g., about 0.5 .mu.m--is formed in the photoactive regions of the substrate. Neither diffusion layer is formed on a texturized surface. Matsutani et al also disclose in FIG. No. 3 of their patent an arrangement in which the entire upper surface of the substrate has a deep junction--viz., 3.0 .mu.m--formed therein and, thereafter, the photoactive regions are "slightly etched" (Col. 2, line 58) to partially remove the upper portion of the deep junction and to leave only the lower portion of the junction--i.e., the lower 0.5 .mu.m is left--thereby creating a shallower junction where the deep junction has been etched. However, FIG. 3 of the patent illustrates a resulting cell in which the etched surface is planar, suggesting the use of a non-texturizing etchant to remove a portion, but not all, of the deep junction in the inter-electrode photoactive regions of the substrate.
In the Gonsiorawski patent, the patentee applies a doped SiO.sub.2 layer to the substrate, etches the doped SiO.sub.2 layer away only in those areas where electrodes are to be affixed, and then subjects the substrate with its doped SiO.sub.2 mask to a deep diffusion process. As a result, a deep junction is formed in the substrate in those areas where the doped SiO.sub.2 layer has been removed, while a shallow junction (said to be from 0.1 .mu.m to about 0.5 .mu.m) is formed in those regions under the doped SiO.sub.2 mask. Such shallow doped regions are, of course, not texturized.
Consequently, insofar as presently known, while the prior art is replete with numerous proposed processes and techniques purported to improve light collection and/or conversion efficiencies, prior to the advent of the present invention there has been no known effective and reliable method for optimizing the diffusion properties in both the contact areas and the photogeneration areas of a light transducer substrate by formation of a deep junction with high surface concentrations of dopant in discrete, narrow, raised, mesa-like areas of the substrate suitable for application of fine-line electrodes by vacuum deposition and/or plating techniques, while forming a shallow junction, preferably on a textured surface, in the photoactive regions of the transducer.
Another problem that has continued to plague the industry has been that of providing light-to-electrical energy transducers such, for example, as solar cells and, especially, concentrator solar cells, which are characterized by their thermal stability--i.e., by their ability to be heated to very high temperatures for extended periods of time without catastrophic loss of output power. A typical instance where thermal stability is important is in those cases where the cell must be glass encapsulated to enhance environmental stability against corrosion, sandblasting, etc.; and, under such requirements, the cell must have sufficient thermal stability as to be capable of withstanding molten and/or semi-solid or soft glass at temperatures commonly ranging from 900.degree. C. to 1000.degree. C. during the glass encapsulation process. Another application where thermal stability is an important characteristic is in the area of space applications where solar cells are continuously exposed to high-energy solar particle fluxes which tend to degrade the crystalline perfection of the cell over a period of time and thereby reduce the cell's power output by factors ranging from 50% to 80%. While in-situ laser annealing has been found to restore a great deal of the crystallinity, such cells commonly suffer from shorting of the junction resulting from migration of electrode metals through the junction into the substrate, thereby rendering the cell virtually useless. Solar cells made in accordance with the present invention, however, permit of optimization of the requisite thermal stability characteristics by virtue of optimization of the junction characteristics in both (i) the inter-electrode photoactive regions of the cell and (ii), the contact regions of the cell; and, this desirable result can be achieved with cells having either texturized inter-electrode photoactive surfaces or polished inter-electrode photoactive surfaces.